Cadmium pigments are stable inorganic coloring agents and are useful in high-temperature processing. Cadmium pigments primarily are used in plastics but also are used in some coating and ceramics. Cadmium pigments are based upon the compound cadmium sulfide (CdS), which produces a golden yellow pigment. Partial substitution of cadmium by zinc or mercury and substitution of sulfur by selenium form a series of compounds in a range of lemon-yellow, orange, red, and maroon. Cadmium sulfide is the primary ingredient of various types of high-temperature pigments.
Chemically, there are two types of cadmium sulfide, cubic CdS and hexagonal CdS. The high-temperature stable hexagonal CdS is the only one used in the pigment industry. Hexagonal CdS can be obtained by calcinating cubic CdS at about 600.degree. C. and also by sulfide precipitation from halide solutions. However, the resulting hexagonal CdS from halide solutions cannot be washed free of halides. Each of the current U.S. pigment manufacturers has developed various proprietary methods for generating pigments with particular color shades and properties, but the production of all cadmium pigments is structured around one generic process which consists of dissolution of cadmium metal or cadmium oxide in acid, sulfide precipitation at about 90.degree. C. to form cubic CdS, filtration and washing, drying, calcination at about 600.degree. C., acid washing for removal of soluble cadmium, filtration and washing, drying, blending and grinding. The process is complicated and cost intensive, especially during the calcination stage. Without proper control, the transformation of cubic CdS to hexagonal CdS would not be complete and some of the cadmium sulfide could be oxidized.